bulletproofs: reject points not in the main subgroup

src/ringct/rctOps.cpp

@@ -60,6 +60,17 @@ namespace rct {
 
     //Various key generation functions
 
+    bool toPointCheckOrder(ge_p3 *P, const unsigned char *data)
+    {
+        if (ge_frombytes_vartime(P, data))
+            return false;
+        ge_p2 R;
+        ge_scalarmult(&R, curveOrder().bytes, P);
+        key tmp;
+        ge_tobytes(tmp.bytes, &R);
+        return tmp == identity();
+    }
+
     //generates a random scalar which can be used as a secret key or mask
     void skGen(key &sk) {
         random32_unbiased(sk.bytes);
@@ -200,6 +211,12 @@ namespace rct {
         return aP;
     }
 
+    //Computes aL where L is the curve order
+    bool isInMainSubgroup(const key & a) {
+        ge_p3 p3;
+        return toPointCheckOrder(&p3, a.bytes);
+    }
+
     //Curve addition / subtractions
 
     //for curve points: AB = A + B